The present invention relates to an apparatus for measuring the thickness of electromagnetic wave absorption surface layer such as water, film, paint or the like applied or attached on the surface of substance such as base plate surfaces, by which the electromagnetic waves such as light rays are caused to reflect toward the apparatus through the layer.
Conventionally, the various apparatuses are proposed as a coated film thickness measuring apparatus which is the surface layer thickness measuring apparatus. For example, Japanese Patent Application Publication (unexamined) Tokkaisho No. 58-115306 is provided. However, according to this system, two units of infrared ray measuring systems are required under the necessity of correction for errors to be caused by the temperature of an object to be measured. Thus, the price of this system becomes higher, and the characteristics of each infrared ray measuring systems are actually different so as to cause the correction error between the systems. Also, the automatic correction for the defect of this system is not performed through not only the variation in the characteristics of the reference infrared ray source, but the time-lapse variation in the electric system and the optical system. Furthermore, although various apparatuses for performing the automatic correction through the time-lapse variation are proposed, they cannot make sufficient corrections both in electric and optical systems.